Virtually the entire catalogue of DRM-free good old games at GOG.com is on sale for up to 50% off through the new year, and they are adding a free copy of Tyrian 2000 to everyone's order as a virtual stocking stuffer. The Humble Indie Bundle #2 is now available, offering the chance to pay what you want for a bundle of Braid, Cortex Command, Machinarium, Osmos, and Revenge of the Titans, once again allowing you to give some of your contribution to charity. Amazon has a boatload of console games on sale (thanks Batman via Gamerdeals).
Two or more eligible games on the EA Store are 50% off. GamersGate has a buy four, pay for three promotion. There are a bunch of strategy games on sale on Matrix Games.